Volcanic Eruptions

William Holmes Spaulding Photograph Collection: Photographic postcard of Lassen Peak in eruption
The 1914-1917 eruption of Lassen Peak was the most recent eruption in the continental United States until the 1980 eruption of Mt St Helens. William Holmes Spaulding Photograph Collection: Photographic postcard of Lassen Peak in eruption.

Photo by R.E. Stinson.


Volcanic eruptions are among the most awesome of all natural phenomena on Earth. They may be strangely beautiful as fountains of glowing-red lava rise above a vent to feed a lava flow that spreads rapidly downhill. Or they may consist of terrifying explosions that send clouds of scorching hot ash high into the atmosphere or roaring down a volcano’s slopes and destroying everything in its path.

While a great range in the type, style, and violence of volcanic eruptions exists, they all are part of one of the most fundamental geologic processes that builds and shapes Earth’s crust.

The place to begin an exploration of the diversity of the different types and kinds of volcanic eruptions is with the definition.

A volcanic eruption is the expulsion of gases, rock fragments, and/or molten lava from within the Earth through a vent onto the Earth’s surface or into the atmosphere.

Illustration of the basic process of magma formation, movement to the surface, and eruption through a volcanic vent.
Illustration of the basic process of magma formation, movement to the surface, and eruption through a volcanic vent.

USGS illustration.

Some volcanic eruptions consist of mostly gas emissions, others are relatively quiet discharges of fluid lava, and yet others are cataclysmic explosions.

Different kinds of eruptions leave characteristic deposits that, in turn, build different types of volcanoes. A magma’s composition, viscosity, and gas content, the eruption rate, and the size of the magma reservoir determines many aspects of eruptions, including how explosive they are.

National parks are great places to observe current volcanic activity.

  • Kilauea in Hawai’i Volcanoes National Park was active from 1983 to 2018, and has had shorter periods of eruption since then.
  • Katmai National Park and Preserve in Alaska is one of the world’s most active volcanic areas with 10 volcanoes that have had historic eruptions.
  • The most recent eruption at the Lassen volcanic center in California occurred in 1917.

A number of other national parks contain volcanoes that have had prehistoric eruptions. Volcanoes in many other parks erupted in the even more distant past. Today, these mountains evoke peace and serenity that belies the violence in their history.

  • Mount Mazama exploded about 7,550 years ago to form Crater Lake.
  • Capulin Volcano erupted just over 54,000 years ago.
  • The Valdez Caldera erupted 1.25 million years ago.

Whether active or ancient, volcanic landforms found in national parks result from their eruption dynamics, with the volcanoes themselves and the lava flows and other deposits they leave behind serving as tangible evidence of the volcanic processes that formed them.

Generation and Rise of Magma

The melted rock (magma) that is erupted in volcanoes does not come from the Earth’s core or even from deep within the mantle. There are also no permanent pools of melted rock found within the crust or mantle. Instead, magma produced from partial melting of the upper mantle.

The heat that causes the partial melting comes from several sources; most importantly, from decay of radioactive elements, such as uranium, thorium, and potassium.

Once magma is generated, it rises buoyantly because it is lighter than the surrounding rock. It moves in slow-moving balloons or diapirs; or in planar fractures (dikes). Sometimes it pools at the base of the crust, and other times it continues to rise to form magma chambers.

Shallow magma chambers may form in regions of neutral buoyancy, i.e., areas where the pressure within the magma body equals the pressure outside it. Magma typically begins to crystallize while they are being stored in magma chambers, forming a liquid-crystal mush.

Eruptions from a shallow magma body may be triggered by injection of more magma into the chamber, by overpressure from increased volatile (gas) content, or by some other factor.

Composition & Viscosity

Volcanic eruptions are inherently physical processes given that they are the emission of gas, magma, and rock from within the Earth. Yet many aspects of eruptions are actually controlled by magma chemistry. In fact, the composition of the magma, as well as its gas content, largely determines whether an eruption is explosive, and the magnitude of that explosivity.

Magma composition impacts nearly everything about a volcano, with viscosity of the melt being one of the most important factors that determines eruption dynamics, and even the shape of a volcanic edifice. Viscosity is the internal friction, or resistance to flow—or how “thick and sticky” or “thin and runny” a lava is.

  • Lavas with low viscosity such as those erupted at Kilauea in Hawai’i Volcanoes National Park flow easily, with flow fronts that move up to 6 miles (10 km) per hour. Speeds in channels or lava tubes on steep slopes can be as fast as 19 miles (30 km) per hour.
  • Highly viscous lavas do not spread out to form wide lava flows, but instead form steep-sided domes immediately above a vent, such as the dome at Novarupta in Katmai National Park.
silicate diagram
Silica tetrahedron diagram.

Modified from NASA illustration.

Silica (SiO2) content has the greatest impact on magma viscosity. Most igneous rocks are made predominately of silica with concentrations ranging from about 45 to 78% by weight. Specifically, silica is arranged in tetrahedrons (Si04 complexes). Silica tetrahedrons can share oxygen atoms to form chains or networks in a melt. Higher concentrations of silica leads to longer and more complex chains.

The greater abundance of complex chains of silica tetrahedrons have a greater propensity to tangle with one another. This impedes their ability to flow past one another, somewhat akin to tangling of long strands of spaghetti.

chain of silica tetrahedrons
Chain of silica tetrahedrons.

Modified from NASA illustration.

Therefore, the general rule is that magmas with high silica content are highly viscous, and ones with low silica have low viscosity (e.g., are inviscid). The presence of other elements, particularly sodium and potassium, can lower viscosity in rhyolitic magmas because they interfere with silica’s ability to form complex chains. Similarly, the presence of water in the melt (which is common) can also decrease viscosity.

In general the viscosity of a low silica magma like basalt is thousands of times more viscous than liquid water. High silica melts can be many orders of magnitude more viscous than basalts.

Their relatively low viscosities are why basalts are generally extruded in quiet (effusive) or mildly explosive eruptions. On the other hand, eruptions of high silica magmas are likely to be explosive (due to both high viscosity and higher gas content).

lava river
Lava flows during the 1984 eruption of Mauna Loa. Hawai'i Volcanoes National Park, Hawai'i.

NPS photo by B. Seibert.

distant view of erupting volcano
The explosive eruption of Katmai's Furpeaked volcano as observed from Homer, Alaska on September 17, 2006. Katmai National Park and Preserve, Alaska.

Photo by Lanny Simpson, Alaska High Mountain Images.

Gas content & exsolution

Scoria with vesicles.

Courtesy of James St. John (Flickr)

Magmas typically contain small amounts of dissolved gas (volatiles). Water and carbon dioxide are the most common volatiles, although sulfur dioxide, hydrogen sulfide, and others may be present.

Until a magma nears the Earth’s surface, the enormous pressure of the overlying rock keeps gases dissolved. Near the surface, the pressure decreases and they can exsolve from the melt, ultimately forming gas bubbles in a process called vesiculation. This exsolution of magmatic gases as a magma ascends towards the surface is one of the forces that propels volcanic eruptions.

The release of pressure as a magma nears the Earth’s surface is similar to the release of pressure in a carbonated beverage when it is opened. The exsolution of carbon dioxide from soda pop due to the release of confining pressure when the can is opened is like the expansion and exsolution of gases that propels eruptions.

Higher volatile contents increase the likelihood of explosive eruptions compared to eruptions of magma with lower concentrations of gases.

Viscosity is also important because gases can escape more easily from thin fluids than thick ones, as can be observed in the spattering that can happen when making jam (a more viscous liquid) versus the simple boiling of water.

In general, the higher the viscosity and the higher the gas content, the more explosive the eruption will be.

ash cloud over volcanic mountain peak
April 21, 1989 eruption of Redoubt Volcano in Lake Clark National Park and Preserve, Alaska.

USGS photo.

  • In the eruptions that build some cinder cones, the vesiculation of basaltic magma from expanding and exsolving gas throws blobs of magma perhaps tens to hundreds of feet into the air that then cool and fall around the vent as cinders.
  • In highly explosive eruptions of silicic magmas, vesiculation can completely shatter the erupting material into tiny bits called volcanic ash in columns that rise tens of thousands of feet into the atmosphere. For example, the April 21, 1989 eruption of Redoubt Volcano in Lake Clark National Park and Preserve formed an eruption column that ascended to a height of 62,000 feet (19 km).

Rate of Eruption

The rate of eruption can also influence how explosive an eruption is. If magma ascends slowly from deep within the crust, it is possible for the dissolved gases to escape nonviolently over time. But when magma ascension and eruption rate is rapid, the dissolved gases must escape all at once and the eruption is more explosive.

Likewise, a soda gently fizzes when the gases dissolved in it are slowly released. Yet it explodes violently when carbon dioxide exsolves rapidly, such as happens after a can is shaken.

It is the sudden release of energy by gas under pressure by rapid exsolution that is one of the main drivers of explosive eruptions.

Size of Magma Reservoir

The size of the magma body beneath a volcano has a strong controlling factor in the magnitude of eruptions because the availability of magma can strongly constrain its size. Small magma bodies simply cannot sustain large eruptions because there is not enough material available.

Cinder cones, even exceptionally large ones like Sunset Crater Volcano, only tap small magma sources. The volume of erupted rock material erupted In 1085 CE to form Sunset Crater Volcano about was 0.12 cubic miles (0.52 cubic km) in volume in contrast to the largest eruption at Yellowstone 2.1 million years ago that expelled nearly 600 cubic miles (2,450 cubic km) of material.

Volcanic Activity

An erupting volcano may include varying types of activity, along with a range of intensity. Volcanic activity includes earthquakes caused by magma movement, gas emissions, effusive emissions of lava, and cataclysmic eruptions.

Most geologists use the term eruption to encompass a whole period of volcanic activity which is bracketed by quiet intervals. The Smithsonian Institute has set an arbitrary interval of three months of complete inactivity of a volcano to separate one eruption from another.

Eruptions generally consist of eruptive pulses and eruptive phases.

  • Eruptive pulses are single explosions that may last a few seconds to minutes.
  • Eruptive phases consist of numerous eruptive pulses that generate a pulsating eruptive column or lava flow, and may last from a few hours to days.

An eruption may consist of many eruptive pulses and last a few days, months, or even years.

Active, Dormant & Extinct

aerial view of a cinder cone and volcanic landscape
Capulin cinder cones is an extinct volcano. Capulin Volcano National Monument, New Mexico.

NPS photo.

Volcanologists describe volcanoes as being active, dormant, or extinct based on how recently they erupted and whether they are likely to do so again.

  • Active: A volcano is considered potentially active if it has erupted during the last 10,000 years. Some volcanoes may have dormant periods between eruptions greater than 10,000 years, but 10,000 years is a convenient cut-off date for activity and is used by convention. An active volcano is currently erupting, or is one that has erupted in historic time. Even though Mount Rainier hasn’t had a significant eruption for a thousand years, it is considered to be an active volcano.
  • Dormant: A volcano that is not erupting now, but is considered likely to erupt in the future. There is no precise distinction between active and dormant volcanoes. Sometimes dormant volcanoes are described as being potentially active. Mount Rainier and the El Malpais National Monument volcanic field are considered dormant.
  • Extinct: An extinct volcano is one that is not expected to erupt again in the future. Sometimes the determination of whether a volcano is extinct is based on the amount of time since its last eruption. Alternatively, some types of volcanoes such as cinder cones typically only erupt once. For example, Capulin Volcano, a cinder cones, is extinct.

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Last updated: July 18, 2022